Glass Transition and Self-Diffusion of Unentangled Polymer Melts Nanoconfined by Different Interfaces

In nanoconfined thin films, numerous studies have revealed the thickness dependencies of different thermophysical properties, including the glass transition temperature (T_g) and self-diffusion coefficient (D). While quantitative relationships between these properties are well-known for bulk polymers, analogous relationships for nanoconfined polymers are still not clear. Herein, T_g-D relationships are studied under nanoconfinement using spectroscopic ellipsometry for measuring T_g and fluorescence recovery after photobleaching for measuring D. Poly(isobutyl methacrylate) (PiBMA) was selected as a model unentangled polymer, and it was nanoconfined to 14-300 nm thick films. Multilayered geometries incorporating PiBMA were constructed to systematically study the influence of free surfaces (i.e., polymer surfaces exposed directly to air, also called uncapped) and surfaces that were in contact with a secondary polymer (also called capped). This multilayer approach additionally allowed investigation of both relatively weak and strong interactions between the polymer and substrate, depending on the existence of hydrogen bonding. The T_g-D relationship observed in nanoconfined thin films deviated from that in the bulk state (e.g., as described by Williams-Landel-Ferry and Stokes-Einstein, or similar relationships). A model was employed that considered the effects of molecular friction between the different confining interfaces and PiBMA, and it successfully described the deviation from bulk behavior.